A few more DBLockManager fixes and cleanups

A few more DBLockManager fixes and cleanups

* Do not do the connection init step if the same DB handle as
  wfGetDB( DB_MASTER ) is being used to avoid clobbering it.
* Remove begin(), since only one of the subclasses wants
  transactions. That one now uses startAtomic() now.
* Make getConnection() throw an error for bad config instead
  of return null, which was not documented or expected.

Change-Id: Ib09a7972d6569c29e83e329a8f7f9f47a393b896